1. Field of the Invention
This invention relates to an image processing apparatus and method for converting and outputting a digital image comprising a density signal obtained by reading the original of an image.
2. Description of the Prior Art
One example of an illustration technique involves projecting a photographic image on paper, obtaining a diagram of the photograph by tracing positions at which the profile and tone of the photograph change, and painting portions on the diagram having the same color and tone with a pigment or pasting colored paper referred to as pantone to produce an image having an effect entirely different from that of the original photograph.
However, when an image is produced by this technique, the following drawbacks are encountered:
(1) It is necessary that the photograph of an original expressed by a near infinity of colors be re-expressed by a limited number of colors. This requires the sense and experience of an illustrator and is difficult for a novice to achieve.
(2) The tracing work and the work required for painting or pasting of the colored paper is an extremely detailed manual operation demanding a high level of skill and an exorbitant amount of time.
(3) A color once painted or pasted on is difficult to change later on and great care must be taken so as not to make a mistake during the painting or pasting operation.
Well-known image processing techniques using a computer include electronic photoengraving in the printing industry and professional laboratory techniques which apply computerized photographic image processing in the photographic laboratory industry.
For example, in a case where the original of an image is read by a high-precision scanner such as a drum scanner and a reproduced image is obtained by operating an image output apparatus, such as a laser-beam printer or film recorder, using the resulting signal, an image processor is provided for the intermediate part of the process, and an input density signal is subjected to such processing as a density expression correction (a .gamma. correction), tone setting, color correction and assembly of cuttings. The following effects are obtained:
(1) Faded color film can be restored.
(2) The tones of highlights and shadows can be adjusted and color representation can be emphasized.
(3) Mechanical failures, photographic errors and errors in development can be remedied.
(4) Unnecessary objects (power lines, rubbish, scratches, etc.) in the image can be eliminated or revised.
(5) A creative image can be expressed and the image area can be enlarged to create a new design.
When special processing of this kind is executed, a film original is photoelectrically scanned and read by a high-precision color scanner, color camera tube or color solid-state image sensor (e.g., a CCD), and processing is performed in dependence upon the density signal of the film original or a digital image signal such as a luminance signal.
Examples of such special processing include systematically arranged mosaic processing, solarization processing, in which a .gamma. curve is made into something unrealistic or discontinuous, and posterization processing.
For example, consider posterization processing, which is comparatively close to an illustration. First, a characteristic feature of an illustration is that the number of colors is limited. In this sense, posterization processing, which reduces the number of tones, satisfies the condition.
More specifically, when a color image is dealt with by a computer, ordinarily the image is expressed by a combination of data composed of the three primary colors red (R), green (G) and blue (B). In posterization processing, processing is executed to reduce each of the R, G and B items of data from 256 tones to 5 tones, by way of example. In this case, a number of colors of greater than 16,000,000 in the original image is reduced to 125 (5.sup.3).
In the example of the prior art described above, however, the following drawbacks are encountered:
First, FIG. 2(a) illustrates, as one example of an original image, the relationship between density data and position in a case where the color orange becomes brighter little by little from left to right. When this data is subjected to posterization processing in accordance with the input/output relationship shown in FIG. 2(b), the R and G items of data are converted as shown in FIG. 2(c). At this time the ratio of R and G undergoes a large change at the positions A, B, C and D. As a result, the color tone changes.
More specifically, the image becomes yellow from the left side up to A, near red from A to C, orange from C to D, and near red again when D is surpassed. The case of an actual illustration, the atmosphere is different from that of an image obtained by the posterization processing of light in order to select colors, such as a pale orange color, intermediate orange color and dark orange color, even if the number of colors is limited.
Furthermore, if the scene serving as the subject is the face of a human being, for example, a pseudo-profile may occur on the inner side of the face with ordinary posterization or with the method of reducing the number of colors. This will not provide a desirable expression of an illustration.